•Eight countries, including Nigeria, UK, US and Australia, took part in the study
•60-80% of disease risk is based on our genetics
More than 40 genes have been linked to Alzheimer's disease for the first time, in a "landmark" study offering hope for better diagnosis and treatment.
Scientists in eight countries, including specialists at Cardiff University, looked at the genetic material of 111,000 people with Alzheimer's.
Findings suggest it is caused by many factors with evidence for a specific protein involved in inflammation.
Study co-author Prof Julie Williams said it was "a major leap forward".
"Genetics has and will continue to help us identify specific disease mechanisms which we can target therapeutically," said Prof Williams, center director at the UK Dementia Research Institute at Cardiff University.
Alzheimer's disease is the most common cause of dementia and affects more than 850,000 people in the UK.
The research team hopes that, in the future, genetic testing will identify those most at risk of developing Alzheimer's before symptoms appear.
The study, published in Nature Genetics, identified 75 genes associated with an increased risk of developing Alzheimer's disease, including 42 genes not previously implicated in the condition.
It also confirmed previous findings regarding the proteins amyloid-beta and tau, which build up in and around nerve cells as Alzheimer's progresses, and found that inflammation and the immune system play a role in the disease.
A group of 111,326 people with Alzheimer's disease were compared with 677,663 healthy individuals to look for differences in their genetic make-up.
Prof Williams said: "This piece of work is a major leap forward in our mission to understand Alzheimer's, and ultimately produce several treatments needed to delay or prevent the disease.
"The results support our growing knowledge that Alzheimer's disease is an extremely complex condition, with multiple triggers, biological pathways and cell types involved in its development."
She added: "Components of our immune system have a big role to play in the development of the disease.
"For example, immune cells in the brain known as microglia are responsible for clearing out damaged tissue, but in some people that may be less efficient which could accelerate the disease.
'Disease risk based on genetics'
"Lifestyle factors such as smoking, exercise and diet influence our development of Alzheimer's, and acting to address these now is a positive way of reducing risk ourselves.
"However, 60-80% of disease risk is based on our genetics and therefore we must continue to seek out the biological causes and develop much-needed treatments for the millions of people affected worldwide."
Dr Susan Kohlhaas, director of research at Alzheimer's Research UK, said: "It's going to take a concerted and global effort to develop life-changing treatments, but this seminal study also gives us hope that research will win, and it gives us the opportunity to work on new treatment targets."
Eight countries, including Nigeria, UK, US and Australia, took part in the study led by Prof Jean-Charles Lambert, research director of Inserm, the French National Institute of Health and Medical Research with Prof Williams and colleague Dr Rebecca Sims in the UK.